PH is very important in my opinion.
Fish that are older and from the wild are not nearly as adaptable as younger or bred fish.
Short term variations in PH within a range of say 1.0 does not seem to have any impact on fish at all, in my time with piranha, but prolonged exposure to either too high or too low ph values causes severe problems with older fish that I have imported.
Many fish keepers that maintain discus and many other soft water fish have no problems at all maintaining their tanks at optimal PH levels for discus, which I feel are far more sensitive than piranha, and these PH levels are generally much lower than needed for piranha.
I do agree with what others have said about LARGE PH swings and they should be avoided at all costs, but I also feel that if you are the reason the fish is not in the wild maybe you should take as much care to provide the same/similar water conditions as the fish would have in the wild, obviously the most suitable water for the fish.